Advanced Poll Header problem error

Hi,

I have recently started using advanced poll. I like it as its easy to use apart from I have no come across an error which only appears once a person has voted.
I have changed my site from a .htm to a .php so that I may display the poll within the page and have this error displaying.

Warning: Cannot modify header information - headers already sent by (output started at c:\inetpub\wwwroot\hsnetwork\index.php:6) in c:\inetpub\wwwroot\hsnetwork\polls\poll\poll\db\poll_cookie.php on line 21

Could you please help.

Thank you in advance.